1. Field of the Invention
This invention pertains to a type of objective lens unit for reflective fluorescence and a type of reflective fluorescence microscope having such an objective lens unit. A reflective fluorescence microscope using the objective lens unit can be used to make an observation from the fluorescence from a specimen due to an excitation light used for reflective illumination.
2. Description of Related Art
A reflective fluorescence microscope is one particular type of microscope. In a reflective fluorescence microscope, short-wavelength excitation light, emitted from a mercury lamp or other light source, is reflectively illuminated in the observational field of view on the specimen. The long-wavelength fluorescence emitted from the specimen is observed. In this specification, an objective lens unit having a front lens group and a rear lens group and used in a reflective fluorescence microscope is referred to as an objective lens unit for reflective fluorescence.
In a conventional reflective fluorescence microscope, the excitation light from a light source is reflected by a dichroic mirror toward an objective lens. From an aperture on the image side of the objective lens, the excitation light is incident on the specimen along an optical axis. In this type of conventional reflective fluorescence microscope, when the specimen is reflectively illuminated, the entire objective lens is used as a condenser lens. When near-UV light is used as the excitation light, it is necessary to select an optical material for the objective lens that has a sufficient transmissivity for, and generates little fluorescence ("self-fluorescence") from, the near-UV light. The reason for this is that if the self-fluorescence of the objective lens reaches the image plane of the reflective fluorescence microscope, then the contrast of the fluorescent image of the specimen, which is the target of observation, is significantly degraded.
Fluorite and fused silica glass are known optical materials having high transmissivity for UV light. High-purity, fused silica glass generates little fluorescence from UV light, but fluorite generates strong fluorescence from UV light. For fluorite, substances have been developed which are characterized by little self-fluorescence generation. On the other hand, for conventional optical glasses, the amount of self-fluorescence generated is several times, and thousands of times in some cases, that of the aforementioned feed materials. Consequently, if only high-purity, fused silica glass and fluorite with prescribed characteristics are used to form the objective lens, then it is possible to realize an objective lens with good UV transmissivity and with little self-fluorescence. Also, the physical characteristics of fused silica glass are much more stable than those of fluorite, and fused silica glass also can be procured easily.
Both fluorite and fused silica glass with good UV transmissivity have small refractive indices. The dispersion rates of these materials are also similar. Consequently, when these two types of optical materials alone are used, it is very difficult to form an objective lens which is properly corrected for chromatic and other aberrations and which has a large numerical aperture. Optical materials other than fluorite and fused silica glass with high dispersion power must be incorporated in the objective lens to properly compensate for chromatic and other aberrations and to provide for a large numerical aperture.
As explained above, for a conventional objective lens for reflective fluorescence, it is necessary to use fluorite and fused silica glass. These materials have high excitation light transmissivities and generate little self-fluorescence. However, when the objective lens is made of fluorite and fused silica glass alone, various aberrations cannot be corrected very well. On the other hand, if optical materials having high dispersion power are included to properly correct for the various aberrations in the objective lens unit, then the total amount of self-fluorescence generated by the objective lens increases. This is a disadvantage.